Hydro-mechanically coupled electric power steering system

ABSTRACT

A power steering system provides improved steering feel whenever negligible power assist is required is required such as during on-center operation or at very high vehicular speeds. The power steering system includes fluid lines used for directly coupling a motor driven pump to a power cylinder. The fluid lines are coupled to a system reservoir or to one another whenever a primary control signal indicative of steering wheel torque has a value below a selected threshold value thereby substantially decoupling the power cylinder from the pump.

This application claims priority to U.S. Provisional Application Ser.Nos. 60/602,027, filed Aug. 16, 2004 and 60/672,387, filed Apr. 18,2005.

BACKGROUND OF THE INVENTION

The present invention relates generally to power steering systems forvehicles, and more particularly to hydro-mechanically coupledelectrically powered steering systems.

Currently it is anticipated that an overwhelming majority of vehicularpower steering systems will be electrically powered in the future. Mostcommon will be electric power steering systems (hereinafter “EPSsystems”) wherein motors deliver torque as a function of current appliedto them by a controller. One example is described in U.S. Pat. No.6,152,254, entitled “Feedback and Servo Control for Electric PowerSteering System with Hydraulic Transmission,” issued Nov. 28, 2000,which is hereby incorporated by reference in its entirety. In that EPSsystem differential pressure is directly delivered to a double-actingpower cylinder from a motor driven reversible fluid pump.

The EPS system described in the '254 patent may at times reflect motorinertia from the system back to the vehicle's steering wheel whenevernegligible power assist is required, such as during on-center operationor at very high vehicular speeds. This may be made worse because themotor inertia is compliantly coupled to the steering wheel via acompliant member such as a torsion bar.

Additionally, new steering applications have been presented whereinon-center pressure offsets will be required for the purpose of negatingnominally steady road crown and/or side wind induced steering loads.This is a problem because the hydraulically coupled EPS system describedin the '254 patent includes a two-position, three-way (shuttle) valveutilized for the purpose of coupling the lower pressure ports of thepump and power cylinder to system reservoir pressure. It has been foundthat provision of even the small amount of fluid required for displacingthe two position, three-way shuttle valve can result in an undesirableimpulse to the host vehicle's steering wheel whenever there is asubstantial on-center pressure offset. This is because reversal ofdifferential pressure polarity then occurs within at least a transitionregion between on-center and linear operation. In greater detail, thepump must speed up to displace the two position, three-way shuttle valveand then comes to an abrupt reduction in speed when the two position,three-way shuttle valve is seated at its new location. This results in afluid pressure spike that is transmitted to the steering wheel via thepower cylinder, rack-and-pinion interface, and steering shaft.

SUMMARY OF THE INVENTION

A hydro-mechanically coupled power steering system according to thepresent invention, provides a significant improvement to the EPS systemwith hydraulic transmission described in the '254 patent. The EPS systemwith hydraulic transmission includes first and second fluid lines thatdirectly couple a motor driven pump to a power cylinder included in asteering gear. The system improves steering feel whenever negligiblepower assist is required such as during on-center operation or at veryhigh vehicular speeds by substantially decoupling the power cylinderfrom the pump.

In exemplary embodiments, the first and second fluid lines are coupledeither to a system reservoir or to one another whenever a primarycontrol signal indicative of steering wheel torque has a value below aselected threshold value. This substantially decouples the powercylinder from the pump. Decoupling the power cylinder from the pumpserves to improve steering feel whenever negligible power assist isrequired because it eliminates the reflected motor inertia from thesystem. The power cylinder may subsequently be progressively re-coupledto the fluid pump as the steering wheel torque increases. Generally, thethreshold value is selected to be an increasing function of vehicularspeed and may even be increased without bound at very high vehicularspeeds.

In another aspect of the present invention, a solenoid-controlled valveapparatus (or valve assembly) is presented for accommodating reversalsof differential pressure polarity by being electrically driven ratherthan being hydraulically driven (e.g., without utilizing any pumpedfluid). In addition, improved fresh fluid replenishment is provided by apair of check valves utilized in conjunction with thesolenoid-controlled valve apparatus. Further, the solenoid-controlledvalve apparatus and check valves in this embodiment reduce the number ofparts by replacing a solenoid-controlled two-position relief valve,suction line, (power cylinder mounted) check valves, and the twoposition, three-way shuttle valve.

In addition to issuing a solenoid-controlling signal to either of thefirst and second solenoids, the system controller issuesmotor-controlling signals to the motor utilized for driving the pump.The motor controlling signals are issued in dependence upon an appliedtorque signal indicative of steering torque applied to the hostvehicle's steering wheel as generated by at least one of redundantapplied torque sensors as well as feedback signals indicative of fluidpressure present in the first and second fluid lines provided byrespective first and second pressure transducers. Thus, the pump iscaused to deliver appropriately pressurized fluid to one of first andsecond ports of the power cylinder while the other one of the first andsecond ports is fluidly coupled to the system reservoir. In the unlikelyevent of an unexpected system fault, both the motor-controlling andsolenoid-controlling signals are faulted to ground potential with theresults that the pump stops and the improved hydraulically coupled EPSsystem immediately goes into a fail-safe mode wherein both of the firstand second fluid lines are fluidly coupled to the system reservoir asexplained above. Thus, the hydraulically coupled EPS system iscontrolled in the general manner taught in the '254 patent.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows.

FIG. 1 is a schematic view representative of an examplehydro-mechanically coupled power steering system according to thepresent invention.

FIG. 2 is a schematic view representative of a second embodiment of anexample hydro-mechanically coupled power steering system of the presentinvention.

FIG. 3 is a schematic plan view representative of a third embodiment ofan example hydro-mechanically coupled power steering system of thepresent invention;

FIGS. 4A, 4B and 4C are sectional views of the solenoid-controlledtwo-way valves of FIG. 3 in three different positions; and

FIG. 5 is a plot depicting operational steering characteristics enabledby the hydraulically coupled power steering system of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One example of a hydro-mechanically coupled power steering system 10A isshown schematically in FIG. 1. FIG. 1 depicts most of the elements ofthe hydro-mechanically coupled power steering system of the '254 patent.A steering wheel 11 is connected to the steerable wheels (not shown) bya suitable steering gear 13 engaged (directly or indirectly) with a gearrack 17. A torque sensor 28 is connected to the steering wheel 11 andgenerates an electrical or electronic signal representative of themagnitude and direction of a steering torque applied to the steeringwheel 11. A secondary torque sensor 28′ provides a redundant torquesignal utilized in a fail-safe function for the power steering system10A. The application of an applied steering torque to the steering wheel11, as sensed by the torque sensor 28, results in the application by thepower steering system 10A of an assisted steering force to the steerablewheels.

The power steering system 10A includes a power cylinder 12 connected tothe gear rack 17 (connection not shown) and arranged to apply anassistive force to longitudinal movement of the gear rack 17. The powercylinder 12 has a first (or “left”) port 56 and a second (or “right”)port 58 and may be a double-acting power cylinder 12. Upon the supply ofa pressurized fluid to one of the first port 56 and the second port 58,the power cylinder 12 assists longitudinal movement of the gear rack 17in the associated direction by applying an assistive force to it. Ofcourse, a manual, mechanical steering force is concurrently supplied tothe steerable wheels through the steering gear 13 and rack 17 as well.The total steering force applied to the steerable wheels is the sum ofthe manual steering force and the powered assist provided by the powercylinder 12.

Differential pressure is directly delivered to the power cylinder 12from a pump 14 controlled by a controller 16. The controller 16 mayinclude a microprocessor, memory and suitable computer programming tocontrol the functions as described herein or may be a hardwired controlcircuit. A vehicle sensor, which in this example is a vehicle speedsensor 19, sends a signal indicative of current vehicle speed to thecontroller 16. The pump 14 may be motor driven and reversible. Inaddition, the lower pressure one of the fluid line 18 or the fluid line20 is fluidly coupled to a system reservoir 22 via a three-way shuttlevalve 24. This serves to keep system pressure at its lowest possiblevalue at all times.

The first and second pressure transducers 64 a and 64 b issue respectivefirst and second pressure signals representative of instant pressurevalues present in the fluid lines 18 and 20. As described in the '254patent, the first and second pressure signals are then used by thecontroller 16 in an inner control loop for achieving accurate and stableselected differential pressure values in the power cylinder 12 independence upon instant torque signals from the torque sensor 28,vehicle speed and any other desired parameter. The secondary torquesensor 28′ provides a redundant torque signal utilized in a fail-safefunction for the power steering system 10A.

Fluid lines 18 and 20 are fluidly coupled to the system reservoir 22 bya valve 26 a as controlled by the controller 16 when a primary controlsignal indicative of steering wheel torque issued from torque sensor 28has a value below a selected threshold value. The valve 26 a may be aproportionally-controlled spring-loaded compound two-way valve 26 a. Thefluid lines 18 and 20 are progressively de-coupled from the systemreservoir 22 as the primary control signal indicative of steering wheeltorque increases. In one example, the threshold value is selected to bean increasing function of vehicular speed and may in fact be increasedwithout bound at relatively high vehicular speeds. The resultingdecoupling of the power cylinder 12 from the pump 14 serves to improveon-center steering feel whenever negligible power assist is required,such as during on-center operation or at very high vehicular speeds.This is because the decoupling enables elimination of the reflectedmotor inertia from the system.

Optionally, the fluid lines 18 and 20 may be coupled to the reservoirbased upon the torque dropping below a first threshold and may bedecoupled from the system reservoir 22 based upon the torque exceeding asecond threshold, equal to or different from the first threshold. Thefluid lines 18 and 20 may be progressively coupled to the systemreservoir 22 and progressively decoupled from the system reservoir 22 asa function of vehicle speed and/or steering wheel torque.

In the illustrated example, inclusion of the valve 26 a permitselimination of a relief valve, suction line, and a pair of check valvesfrom the EPS system with hydraulic transmission (as described in the'254 patent). The relief valve was used as a fail-safe device to coupleboth fluid lines 18 and 20 to the system reservoir 22 should a systemfailure occur. The spring-loaded feature of the valve 26 a biases thevalve 26 a to an open position as an operational fail-safe feature. Inthe open position, the fluid lines 18 and 20 are fluidly connected tothe system reservoir 22 in the event of any system failure. Thus, thevalve 26 a performs the function of the relief valve. Since the fluidlines 18 and 20 are independently coupled to the system reservoir 22 bythe valve 26 a via ports 30 and 32, the valve 26 a serves to introducefresh reservoir fluid under steering recovery situations as well.

A second embodiment of a hydro-mechanically coupled power steeringsystem 10B is shown in FIG. 2. In the power steering system 10B, thefluid lines 18 and 20 are fluidly coupled to one another by a valve 26B,which may be a proportionally-controlled spring-loaded two-way valve. Tothe extent not otherwise described or shown, the second embodiment ofthe power steering system 10B and its operation is the same as that ofthe first embodiment in FIG. 1. The valve 26B serves to fluidly couplethe fluid lines 18 and 20 one to another whenever the primary controlsignal indicative of steering wheel torque issued from torque sensor 28has a value below a selected threshold value. The fluid lines 18 and 20are then progressively de-coupled from one another as the primarycontrol signal indicative of steering wheel torque increases in themanner described above.

Optionally, the fluid lines 18 and 20 may be coupled to one anotherbased upon the torque dropping below a first threshold and may bedecoupled from one another based upon the torque exceeding a secondthreshold, equal to or different from the first threshold. The fluidlines 18 and 20 may be progressively coupled to one another andprogressively decoupled from one another as a function of vehicle speedand/or steering wheel torque.

A power steering system 10C according to a third embodiment is shown inFIG. 3. To the extent not otherwise described or shown, the powersteering system 10C and its components operate similarly to those inFIGS. 1-2. In this embodiment, the fluid line 18 or the fluid line 20instantly conveying the lower-pressure fluid is fluidly coupled to thesystem reservoir 22 via a respective one of back-to-backsolenoid-controlled two-way valves 126 and 128 included in asolenoid-controlled valve apparatus 130 (or “valve assembly”). Thisserves to keep system pressure at its lowest possible value at alltimes.

The improvement comes about because it has been found that provision ofeven the small amount of fluid required for displacing the two position,three-way shuttle valve 24 (FIGS. 1-2) can result in an undesirableimpulse to the host vehicle's steering wheel 11 whenever the EPS systemwith hydraulic transmission is operated with an on-center pressureoffset as described in detail below with reference to FIG. 5. This isbecause reversal of differential pressure polarity can then occur withinat least a transition region between on-center and linear operation. Ingreater detail, the pump 14 in FIGS. 1-2 must speed up in order todisplace the valve 24 and then comes to an abrupt reduction in speedwhen the valve 24 is seated at its new location. This results in a fluidpressure spike being transmitted to the steering wheel 11 via the powercylinder 12, rack-and-pinion interface, and steering shaft. By way ofcontrast, the back-to-back solenoid-controlled two-way valves 126 and128 utilized in the solenoid-controlled valve apparatus 130 are switchedvia actuation of respective first and second solenoids 36 and 38, andtherefore are switched without similar fluid consumption.

As further explained in the '254 patent, a relief valve was used as afail-safe device to simultaneously couple fluid lines and to the systemreservoir 22 should a system failure occur. But herein, as will bedescribed in greater detail below, this task is more easily accomplishedby simply de-energizing both of the first and second solenoids 36 and38. Thus, utilization of the solenoid-controlled valve apparatus 130results in elimination of the relief valve, two-position, three-wayvalve, a suction line, and a (power cylinder mounted) pair of checkvalves.

Also, the solenoid-controlled valve apparatus 130 provides thedecoupling function as described with respect to the embodiments inFIGS. 1-2, whenever negligible power assist is required such as duringon-center operation or at very high vehicular speeds by substantiallydecoupling the power cylinder from the pump. The solenoid-controlledvalve apparatus 130 serves to fluidly couple the fluid lines 18 and 20one to another and to the system reservoir 22 whenever the primarycontrol signal indicative of steering wheel torque issued from torquesensor 28 has a value below a selected threshold value. The fluid lines18 and 20 are then progressively de-coupled from one another and fromthe system reservoir 22 as the primary control signal indicative ofsteering wheel torque increases in the manner described above.Optionally, as also described above, the fluid lines 18 and 20 may becoupled to one another and to the system reservoir 22 based upon thetorque dropping below a first threshold and may be decoupled from oneanother and from the system reservoir 22 based upon the torque exceedinga second threshold, equal to or different from the first threshold. Thefluid lines 18 and 20 may be progressively coupled to one another andprogressively decoupled from one another as a function of vehicle speedand/or steering wheel torque.

As depicted in FIGS. 4A, 4B and 4C, the solenoid-controlled valveapparatus 130 may be assembled within a valve body 40 formed such thatit can be positioned and retained in a known manner within a straightthread “O” ring boss 42 formed in a manifold block 44. In any case, theback-to-back solenoid-controlled two-way valves 126 and 128 insolenoid-controlled valve apparatus 130 respectively include first andsecond valve spools 46 and 48 disposed within a common valve bore 50formed in a valve body 40. Retaining rings 52 are provided for retainingthe first and second valve spools 46 and 48 during handling prior toinstalling the first and second solenoids 36 and 38, but in normal usethe first and second valve spools 46 and 48 abut one another at contactnode 54 in the manner shown in FIGS. 4B and 4C.

The solenoid-controlled valve apparatus 130 also includes a compressionspring 57 located by cylindrical bosses 59 and against shoulders 61 ofthe first and second valve spools 46 and 48. The above noted failsafefunction is implemented by stopping the pump 14 and de-energizing bothof the first and second solenoids 36 and 38 whereby the compressionspring 57 urges both of the first and second valve spools 46 and 48toward retracted positions as shown in FIG. 4A. Then fluid can freelypass between the first fluid line 18 and the second fluid line 20 viafirst and second circumferential grooves 63 and 65, first and secondvalve body ports 66 and 68, and annular passage 70 formed in and withinthe valve body 40. This enables emergency manual steering wherein fluiddisplaced by one side of the piston 72 of the power cylinder 12 is ableto freely flow to the other via first and second ports 56 and 58 of thepower cylinder 12, portions of the first fluid line 18 and the secondfluid line 20 included within the manifold block 44, and thesolenoid-controlled valve apparatus 130. This position also provides thefunction described above of decoupling the pump 14 and power cylinder 12whenever negligible power assist is required such as during on-centeroperation or at very high vehicular speeds, since the fluid lines 18 and20 are coupled together and to the system reservoir 22.

During normal operation of the power steering system 10C, one of thefirst or second solenoids 36 or 38 is energized as depicted in either ofFIGS. 4B and 4C. This fully compresses the compression spring 57 as thefirst and second valve spools 46 and 48 are driven into contact with oneanother at the contact node 54. Then the first and second valve spools46 and 48 are driven toward the retracted position of the other of thefirst and second solenoids 36 and 38. Preferably, the first and secondsolenoids 36 and 38 include internal compliant stops (not shown) inorder to cushion the end stopping point. In any case, when the firstsolenoid 36 is energized as shown in FIG. 4B, the first and second valvespools 46 and 48 are driven toward the retracted position of the secondsolenoid 38 whereby the second valve body ports 68 are fluidly connectedto the annular passage 70 and thus to valve body relief ports 78, reliefcircumferential groove 80, and reservoir fluid line 82, whereby thesecond fluid line 20 is fluidly coupled to the system reservoir 22. Onthe other hand, when the second solenoid 38 is energized as shown inFIG. 4C, the conjoined first and second valve spools 46 and 48 aredriven toward the retracted position of the first solenoid 36 wherebythe first valve body ports 66 are fluidly connected to the annularpassage 70 and thus to valve body relief ports 78, reliefcircumferential groove 80, and reservoir fluid line 82, whereby thefirst fluid line 18 is fluidly coupled to the system reservoir 22.

It is of course necessary to fluidly isolate the relief circumferentialgroove 80 from the first and second circumferential grooves 63 and 65.This is accomplished in a known manner via sealing action of a pair ofsealing rings 84 of rectangular cross section disposed incircumferential grooves 86 formed between the relief circumferentialgroove 80 and the first and second circumferential grooves 63 and 65.The O-ring used in conjunction with the straight thread “O” ring boss 42serves to fluidly retain pressurized fluid in the first circumferentialgroove while another O-ring used in conjunction with another “O” ringboss 88 is utilized to fluidly retain pressurized fluid in the secondcircumferential groove. In addition, a nut 90 and washer 92 included inthe “O” ring boss 88 provide a locking function for securing the valvebody 40 fixedly in place within manifold block 44.

Preferably, the first and second solenoids 36 and 38 are formed withremovable coils 94 and fluidly sealed tubes 96 that are similarlyadapted for positioning and retention within straight thread “O” ringbosses 98 formed within either end of the valve body 40. As such, fluidis retained within the valve body 40 by O-rings used in conjunction withthe straight thread “O” ring bosses 98. In addition, internal portionsof each fluidly sealed tube 96 are vented to the same fluid pressurepresent at the contact node 54 and shoulders 61 via passageways 100formed in each of the first and second valve spools 46 and 48.

With reference now again to FIG. 3, the system controller 16 issuesmotor controlling signals to the motor in dependence upon at least anapplied torque signal indicative of steering torque applied to steeringwheel 11 as generated by at least one of the redundant torque sensors 28and 28′ in order to control the power steering system 10C. Preferably,the power steering system 10C also includes respective first and secondpressure transducers 64A and 64B for issuing pressure signals to thesystem controller 16 that are representative of instant pressure valuespresent in the first and second fluid lines 18 and 20. In that case, thesystem controller 16 establishes closed-loop control of differentialfluid pressure delivered by the pump 14 to the first and second ports 56and 58 of the power cylinder 12 while also directing thesolenoid-controlled valve apparatus 130 to fluidly couple the one of thefirst and second ports 56 and 58 of the power cylinder 12 having lowerpressure to the system reservoir 22.

Curve 108 in FIG. 5 is a typical “pressure-effort” curve depictingoperation of the power steering system 10C wherein “on-center,” or zeroapplied steering torque operation is implemented with zero differentialpressure applied to the power cylinder 12 as depicted at point 110. Suchoperation is typical under conditions of no cross wind or appreciableroad crown. On the other hand, the power steering system 10C is alsocapable of operating with zero applied steering torque under conditionsof significant cross wind or appreciable road crown. This isaccomplished via the system controller 16 internally processing apressure offset signal to generate an atypical “pressure-effort” curve112 having an offset or non-zero valued differential pressure on-centeras depicted at point 114. In so doing a driver of the host vehicle willnot sense steering forces supplied by the power steering system 10C inopposition to the presence of a continuing cross wind or appreciableroad crown. The system controller 16 may gradually initiate the offsetbased upon a sensed fairly constant, small torque from the torque sensor28 that exceed a predetermined period of time.

The problem that such offset operation presents, however, is that theassociated transfer of polarity of differential pressure between thefirst fluid line 18 and the second fluid line 20 occurs off-center atpoint 116 whereat the curve 112 has a non-zero slope, and further,whereat the driver is probably moving the steering wheel 11. In order toensure a smooth transfer of one of the first fluid line 18 and thesecond fluid line 20 being fluidly connected to the system reservoir 22to the other, it is preferable to form the first and second valve bodyports 66 and 68, and the shoulders 61 of the first and second valvespools 46 and 48 such that their metering edges 118 and 120 are spacedin a critically lapped fashion whenever the first and second valvespools 46 and 48 are abutted at contact node 54. This ensuressimultaneous fluid decoupling and coupling of the first and second, orsecond and first, fluid lines 18 and 20 with the system reservoir 22 aseither of the first and second solenoids 36 or 38 is de-energized andthe opposing solenoid 38 or 36 energized.

First and second check valves 122 and 124 depicted in FIG. 3 areutilized for fluidly coupling the system reservoir 22 to a first orsecond pump port 226 or 228 whenever either is sufficiently subject to asuction condition. Such suction conditions are induced in either of thefirst or second pump ports 226 or 228 whenever the pump 14 is operatedabove a selected speed. This condition is implemented is implemented viapressure drop at either of orifices 230 placed in the first fluid line18 and the second fluid line 20 between the first and second checkvalves 122 and 124 and the solenoid-controlled valve apparatus 130. Whenoperated in this manner, some of the returning (i.e., from the port 56or 58 of the power cylinder 12 having lower pressure) fluid is thenreturned to the system reservoir 22 via the respective one of theback-to-back solenoid-controlled two-way valves 126 or 128 even as alike flow of fresh replenishment fluid is then supplied to therespective first or second pump port 226 or 228 via the respective oneof the first and second check valves 122 and 124.

Having described the invention, however, many modifications thereto willbecome immediately apparent to those skilled in the art to which itpertains, without deviation from the spirit of the invention. In oneexample, the solenoid-controlled two-way valves 126 or 128 are separatedfrom one another instead of locating them in their preferredback-to-back orientation in the common valve bore 50. And of course, thevalve body 40 could additionally include first and second internalgrooves in communication with the first and second valve body ports 66and 68 with appropriate edges thereof interdicting with the shoulders 61in place of the first and second valve body ports 66 and 68 themselves.Such modifications clearly fall within the scope of the invention. Also,in the examples shown, the driver input is via a steering wheel 11 andthe signal from the sensor represents torque; however, other driverinputs, input signals and input devices could also be used.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A method of operating a hydro-mechanically coupled power steeringsystem having a pump coupled to a power cylinder, the method comprisingthe steps of: a) sensing a steering input from a driver; b) at leastsubstantially de-coupling the pump from the power cylinder based uponthe sensed steering input decreasing below a first threshold value; andc) coupling the pump to the power cylinder based upon the sensedsteering input increasing above a second threshold value.
 2. The methodof claim 1 wherein said step c) further includes the step ofprogressively coupling the pump to the power cylinder as the sensedsteering input increases.
 3. The method of claim 1 wherein said step b)further includes the step of coupling at least one of the pump and thepower cylinder to a system reservoir based upon the sensed steeringinput decreasing below the first threshold value.
 4. The method of claim1 wherein said step b) further includes the step of coupling an input ofthe pump to an output of the pump.
 5. The method of claim 1 wherein thesteering input from the driver is a steering wheel input.
 6. The methodof claim 1 wherein the sensed steering input from the driver is a torqueand wherein the torque is compared to the first threshold value in saidstep b) and to the second threshold value in said step c).
 7. The methodof claim 1 wherein the first threshold value is equal to the secondthreshold value.
 8. The method of claim 1 wherein the power steeringsystem further includes first and second fluid lines coupling the pumpto the power cylinder.
 9. The method of claim 8 wherein said step b)further includes the step of coupling the first line to the second line.10. The method of claim 8 wherein said step b) further includes the stepof coupling at least one of the first and second fluid lines to a systemfluid reservoir.
 11. A power steering system comprising: at least onepump; at least one power cylinder; and at least one valve selectivelyeffectively coupling and decoupling the at least one power cylinder fromthe at least one pump.
 12. The system of claim 11 further includingfluid lines connecting the at least one pump to the at least one powercylinder.
 13. The system of claim 12 wherein the at least one valveselectively couples the fluid lines to one another in order toselectively decouple the at least one power cylinder from the at leastone pump.
 14. The system of claim 12 wherein the at least one valveselectively couples the fluid lines to a system fluid reservoir in orderto selectively decouple the at least one power cylinder from the atleast one pump.
 15. The system of claim 11 further including a driverinput sensor, the at least one valve selectively coupling and decouplingthe at least one power cylinder to the at least one pump based upon asignal from the driver input sensor.
 16. The system of claim 15 whereinthe driver input sensor is a torque sensor.
 17. The system of claim 16wherein the at least one valve includes a proportionally-controlledspring-loaded compound two-way valve.
 18. The system of claim 16 whereinthe at least one valve includes at least one solenoid.
 19. The system ofclaim 16 wherein the at least one valve includes first and secondsolenoid-controlled two-way valves.
 20. The system of claim 11 furtherincluding a vehicle sensor, the at least one valve selectively couplingor decoupling the at least one power cylinder to the at least one pumpbased upon a signal from the vehicle sensor.
 21. The system of claim 20further including a driver input sensor, the at least one valveselectively coupling and decoupling the at least one power cylinder tothe at least one pump based upon the signal from the vehicle sensor andbased upon a signal from the driver input sensor.
 22. The system ofclaim 21 wherein the vehicle sensor is a vehicle speed sensor.
 23. Thesystem of claim 22 wherein the at least one valve progressively couplesthe at least one power cylinder to the at least one pump based upon atleast one of the signal from the driver input sensor and the signal fromthe vehicle speed sensor.
 24. The system of claim 21 wherein the driverinput sensor is a torque sensor.
 25. The system of claim 11 wherein theat least one valve includes at least one solenoid.
 26. The system ofclaim 11 wherein the at least one valve includes first and secondsolenoid-controlled two-way valves.
 27. A hydraulically coupled powersteering system comprising: a pump; a power cylinder; a first fluid linecoupling the pump to a first port of the power cylinder; a second fluidline coupling the pump to a second port of the power cylinder; a systemreservoir; and a valve assembly including first and secondsolenoid-controlled two-way valves for selectively fluidly coupling thefirst fluid line and the second fluid line to the system reservoir. 28.The system of claim 27 further including: at least one driver inputsensor for generating a driver input signal in response to a driverinput; and a system controller selectively actuating the first andsecond solenoid-controlled two-way valves based upon the driver inputsignal.
 29. The system of claim 28 wherein the at least one driver inputsensor includes at least one torque sensor measuring an applied torqueto a steering wheel and generating an applied torque signal.
 30. Thesystem of claim 29 wherein the system controller selectively controlsfluid pressure to one of the first port and the second port of the powercylinder based upon the applied torque signal while controlling thevalve assembly to couple the other of the first port and the second portof the power cylinder to the reservoir.
 31. The system of claim 30wherein the first and second solenoid-controlled two-way valves includesfirst and second valve spools disposed back-to-back within a common boreformed in a valve body.
 32. The system of claim 31 wherein the first andsecond valve spools having metering edges, the valve body havingmetering edges, wherein the metering edges of the first and second valvespools are spaced in a critically lapped fashion with reference to themetering edges of the valve body such that when either of the first andsecond solenoids is energized while the other is de-energized, theenergized solenoid simultaneously urges both of the first and secondvalve spools toward a position in which one of the first and secondfluid lines is decoupled from the system reservoir and the other of thefirst and second fluid lines is coupled to the system reservoir.
 33. Thesystem of claim 32 wherein the first and second valve spools are urgedapart by a spring such that both of the first and second fluid lines arecoupled to the reservoir when both of the first and second solenoids arede-energized.
 34. The system of claim 27 further comprising first andsecond pressure transducers generating pressure signals representativeof pressure in the first and second fluid lines, respectively, thecontroller establishing closed-loop control of differential fluidpressure delivered by the pump to the first and second ports of thepower cylinder and directing the valve assembly to fluidly couple alower-pressure one of the first and second ports of the power cylinderto the system reservoir.
 35. The system of claim 27 further comprising afirst check valve for fluidly coupling the system reservoir to a firstpump port based upon the first pump port having a suction condition inexcess of a first threshold.
 36. The system of claim 35 wherein thefirst fluid line is connected to the first pump port, the system furtherincluding a first orifice in the first line between the first checkvalve and the valve assembly.
 37. The system of claim 36 wherein fluidis supplied to the first pump port via the first check valve based uponthe pump being operated above a threshold rate and based upon the firstpump port having the suction condition in excess of the first threshold,38. A valve assembly comprising: a valve body having a bore therein; afirst port, a second port and a relief port extending through the bodyto the bore; a first valve spool having a metering edge, the first valvespool disposed within the bore for selectively fluidly coupling thefirst port to the relief port; a second valve spool having a meteringedge, the second valve spool disposed within the bore for selectivelyfluidly coupling the second port to the relief port; a first solenoidoperatively coupled to the first valve spool; a second solenoidoperatively coupled to the second valve spool; and the valve body havingmetering edges, wherein the metering edges of the first valve spool andthe second valve spool are spaced in a lapped fashion with reference tothe metering edges of the valve body such that when either of the firstsolenoid or the second solenoid is energized while the other isde-energized, the energized one of the first solenoid and the secondsolenoid urges both the first valve spool and the second valve spooltoward a position in which one of the first port and the second port isdecoupled from the relief port and the other of the first port and thesecond port is coupled to the relief port.
 39. The valve assembly ofclaim 38 further including a spring urging the first valve spool and thesecond valve spool away from one another such that both of the firstport and the second port are coupled to the relief port whenever both ofthe first solenoid and the second solenoid are de-energized.
 40. Thevalve assembly of claim 38 wherein the first valve spool and the secondvalve spool are positioned between the first solenoid and the secondsolenoid.